1. Field of the Invention
The present invention relates to a bidirectional isolated multi-level DC-DC converter and control method thereof. Particularly, the present invention relates to the bidirectional isolated multi-level DC-DC converter and control method thereof for reducing the dimensions.
2. Description of the Related Art
Generally, a conventional isolated DC-DC converter has been widely used in various industrial fields. Even though the conventional isolated DC-DC converter has a specific advantage of simple and easy control, it has several drawbacks in operation, including low efficiencies, high ripple quantities, high electromagnetic interference and requiring the use of a filter circuit with a large capacity. Conversely, a conventional multi-level DC-DC converter has several advantages of relatively higher efficiencies, relatively lower electromagnetic interference and allowing the use of a filter circuit with a relatively lower capacity even though it has a drawback of requiring sophisticated control.
For example, FIG. 1 shows a schematic view of a conventional multi-level DC-DC converter in accordance with the prior art. Referring initially to FIG. 1, the multi-level DC-DC converter 9 includes a transformer 90 which has a low voltage side (left side in FIG. 1) and a high voltage side (right side in FIG. 1). The multi-level DC-DC converter 9 further includes an inductor 911 and four diodes 912 connected thereto at the low voltage side of the transformer 90. Correspondingly, the multi-level DC-DC converter 9 also includes six high-voltage side switches 92 and two diodes 93 connected thereto at the high voltage side of the transformer 90.
However, the above-mentioned multi-level DC-DC converter 9 has several drawbacks, including a sophisticated structure at the high voltage side of the transformer 90, an increase of total manufacturing cost and dimensions and unidirectional power transmission. In order to simplify the structure, lower manufacturing cost and minimize dimensions, a number of components (e.g. two diodes 93) at the high voltage side of the transformer 90 must be decreased.
For example, U.S. Patent Publication No. 20090059622, entitled “Bi-directional DC-DC converter and method for controlling the same,” discloses a bi-directional DC-DC converter has a transformer for connecting a voltage type full bridge circuit connected to a first power source and a current type switching circuit connected to a second power source. A voltage clamping circuit constructed by switching elements and a clamping capacitor is connected to the current type switching circuit. The converter has a control circuit for cooperatively making switching elements operative to control a current flowing in a resonance reactor.
Another U.S. Patent Publication No. 20120098341, entitled “Bidirectional DC-DC converter and control method thereof,” discloses a small-size, high-efficiency, isolated, bidirectional DC-DC converter. The bidirectional DC-DC converter includes a transformer in which windings are magnetically coupled, switching circuits, a diode which is connected in parallel with a switch, smoothing capacitors, and a control section. First and second DC power supplies, which are connected in parallel with the smoothing capacitors, respectively, provide bidirectional electrical power transfer. When electrical power is to be transferred from the first DC power supply to the second DC power supply, the switch is maintained in the ON state. When, on the other hand, electrical power is to be transferred from the second DC power supply to the first DC power supply, the switch is maintained in the OFF state to prevent a reverse electrical power flow from the first DC power supply.
However, there is a need of improving the bidirectional DC/DC converter disclosed in U.S. Patent Publication No. 20090059622 and U.S. Patent Publication No. 20120098341 for simplifying the entire structure and enhancing the power conversion efficiency. The above-mentioned patent publications are incorporated herein by reference for purposes including, but not limited to, indicating the background of the present invention and illustrating the state of the art.
As is described in greater detail below, the present invention provides a bidirectional isolated multi-level DC-DC converter and control method thereof. A DC-DC converter is adjustably operated in a boost mode or a buck mode in such a way to overcome the problems of the conventional bidirectional DC-DC converter.